Semiconductor-On-Insulator (SeOI) and, in particular, SOI semiconductor devices, are of increasing interest in present and future semiconductor manufacturing, for example, in the context of the Complementary Metal Oxide Semiconductor (CMOS) technology.
A Metal Oxide Semiconductor (MOS) transistor, irrespective of whether an n-channel transistor or a p-channel transistor is considered, comprises so-called pn-junctions that are formed by an interface of highly doped drain and source regions with an inversely or weakly doped channel region disposed between the drain region and the source region. The conductivity of the channel region, i.e., the drive current capability of the conductive channel, is controlled by a gate electrode formed near the channel region and separated therefrom by a thin insulating layer.
Recently, multiple-gate, in particular, double-gate transistors have been introduced in the art. As compared to transistors with a single gate, double-gate transistors exhibit a higher on-current, a lower off-current, a better sub threshold swing and a lower variation of the threshold voltage due to a relatively lightly doped channel region. Double-gate transistors are preferably realized in fully depleted SOI structures with thin buried oxides and highly doped back plane (layer). Double-gate transistors are part of core circuits and periphery circuits of DRAM devices.
In the art, however, the manufacturing process of fully depleted double-gate SOI transistors with highly doped back plane is complicated and may cause damage within the SOI region due to a high dose implant used to form the back plane. Conventionally, the doped back plane is formed by implantation through both the SOI layer and the buried oxide layer. The contamination with dopants in the active layer caused by the back plane implantation, however, results in an increased variation of the threshold voltage. The higher the doping rate that is selected, the higher the resulting variation of the threshold voltage. Moreover, since a relatively high doping energy is necessary in the art in order to form the back gate, deeply extending doping regions result. This badly affects miniaturization of the devices. Thus, improvements in this technology are necessary.